Story time! Long ago, when Lhikan was a Toa, there was a simple blacksmith, that created a sword for Makuta Teridax. It was enhanced by the Mask of Shadows, and it was call; the Adicus Blade of Shadow! The original Adicus Blades were made for the first Toa Team, but Adicus Blades were eventually made for all the Elements, and they're basically like the rings in the Lord of the Rings. When Lhikan found out that Teridax was about to move on the Matoran (the events of Legends of Metru Nui) he stole the Adicus Blade of Shadow, and before giving out the Toa Stones, he gave the Blade to a Ta-Matoran who would resist its power more than any other, a Matoran named Lucan.

Note 1: I did not come up with the Adicus Blades, I just made this one.
Note 2: Makuta Hunter came up with the Adicus Blades.